The present invention relates to cylinder heads adapted to internal combustion engines and particularly small air-cooled engines and more particularly to a cylinder head for air-cooled engines characterized in that a part of the top wall of the cylinder head is depressed downward to form a rocker arm chamber and a cooling wind passage reaching the upper part of a suction port from the side of an exhaust port is formed between the bottom wall of said rocker arm chamber and the upper wall of a cylinder fitting surface.
FIG. 1 is a plan view of a typical example of a conventional generally used air-cooled side valve engine. This kind of generally used side valve engine is so wide in the use and so convenient to mount on a supporting frame and to handle that it is considered preferable to house and arrange an air cleaner a and muffler b of a carburetor between perpendicular planes A--A and B--B in the drawing. Therefore, a valve mechanism of such suction and exhaust systems as a suction port and exhaust port is fitted to a cylinder d so as to be in a direction substantially at right angles with the axial direction Y--Y of a crank shaft c.
Most of the current generally used air-cooled four-cycle engines are of such side valve type. However, such side valve engine has a defect that the engine performance is low. On the other hand, it is well known that a so-called overhead valve air-cooled engine having a valve mechanism of suction and exhaust systems at the top of a cylinder is higher in the engine performance than the above mentioned side valve engine.
However, the above mentioned overhead valve air-cooled engine has defects that the total height of the engine is higher than in the side valve air-cooled engine, therefore the engine mounting height is higher and the handling is difficult. These points shall be explained with reference to FIGS. 2 and 3 schematically showing a conventional overhead valve engine. A rocker arm chamber n housing a rocker arm and valve spring not illustrated is formed on the upper surface of the top of a cylinder head m, a combustion chamber g is formed below the rocker arm chamber n and a screw hole e for a spark plug is provided in the combustion chamber g. Further, an exhaust flange f is provided as illustrated by considering the arrangement of the above mentioned muffler b, carburetor and air-cleaner a. By the way, the engine exhaust has the flowing direction turned at right angles by an exhaust pipe not illustrated and is led to the muffler.
In the drawings, symbol h denotes a fin, i denotes a suction valve port, j denotes an exhaust valve port, k denotes a push rod hole and l denotes a suction flange.
In the air-cooled engine, a cooling wind is led to cool mostly the combustion chamber g, spark plug screw hole e and suction and exhaust valve ports i and j. However, in such case, the cooling wind will cool the screw hole e for the spark plug, will then hit the fin h, will turn the direction as shown by the arrow A in FIG. 2 and will cool the suction and exhaust valve ports i and j and combustion chamber g while passing through a through hole B within the cylinder head. In such case, the cooling wind will be obstructed by the suction port and exhaust port and therefore, if the cooling wind passage area is made large, there will be defects that the height of the cylinder head m will have to be made high and the total height of the engine will be high.